Obsessive compulsive disorder (OCD) is an anxiety disorder that affects about 2.2 million American adults. It is characterized by involuntary, recurrent, obsessive thoughts and repetitive, compulsive behaviors.

Most people living with OCD have both obsessive and compulsive behaviors. Common examples of obsessions — which are usually disturbing and feel uncontrollable — include fear of germs or illness, fear of harming yourself or others, and the idea that everything must be orderly and symmetrical. Common compulsions — which are usually performed to make obsessive thoughts stop — include excessive cleaning, constantly double-checking things like doors and locks, and repetitive behaviors like counting and tapping to reduce anxiety.

OCD is often associated with other mental illnesses and related disorders. It may co-occur or share characteristics with these conditions. The following are five examples.

1. Schizophrenia. A diagnosis of OCD is associated with higher rates of schizophrenia and schizophrenia spectrum disorders, according to a new study published in JAMA Psychiatry. Children of parents with OCD are also more likely to develop schizophrenia and schizophrenia spectrum disorders. More research is needed to identify the risk factors that are common to OCD, schizophrenia, and schizophrenia spectrum disorders, according to the researchers.

2. Depression. One study published in the Journal of Affective Disorders found that major depressive disorder was 10 times more common in people with OCD than in the general population. Symptoms of depression include feelings of helplessness and hopelessness, appetite or weight changes, and loss of interest in daily activities.

Some people with OCD may find that their OCD symptoms get worse when they are experiencing a depressive episode. The co-occurrence of OCD and depression can also have negative effects on treatment. A study published in the Journal of Clinical Psychiatry found that people who only had OCD had better treatment outcomes than people who had OCD and depression.

4. Bipolar disorder. About 5.7 million American adults have bipolar disorder, a serious mental illness characterized by extreme shifts in mood. For people with bipolar disorder, anxiety disorders can be very common. OCD was found to be the most common anxiety disorder for people with bipolar disorder, according to a 2006 study. OCD occurs in up to one-third of bipolar patients, which can complicate diagnosis and treatment. Symptoms of bipolar disorder can vary, and their severity may change over time.

Excessive drug or alcohol abuse may be used to self-medicate, but it can worsen OCD symptoms and put people at risk for dangerous medication interactions. Both substance abuse and OCD can be treated with medications, psychotherapy, and cognitive-behavioral therapy. But it’s important to know the warning signs and ways to get help.